1. Field of the Invention
The present invention relates to a tunneling magnetoresistive effect device, such as a geomagnetism sensor for magnetic field measurement or navigation, and a direction sensor system using the device.
2. Description of the Related Art
Examples of conventional magnetism sensors include magnetoresistive effect devices (MR devices), magnetism impedance devices (MI devices), flux gate sensors, and semiconductor Hall effect sensors. Among these sensors, MI devices, which have been developed only recently, can constitute thin-film and small-sized Ml sensors, and therefore are expected to be improved even further. An MI device can also sense a magnetic field strength from a change in the magnetic field of high-frequency impedance, when high-frequency electric current is applied to the MI device.
In addition to those magnetism sensors, tunneling magnetoresistive effect devices (TMR devices) have been recently developed. A TMR device has a plurality of magnetic thin-film layers, with an insulating layer being interposed in between. In such a TMR device, electrons are transmitted through the insulating layer by a tunneling effect, while maintaining the spin. Here, a magnetic field is sensed based on the tunnel permeability coefficient that is changed with the magnetized state affected by the tunneling effect. Having a very high magnetic field sensitivity, a ferromagnetic tunneling effect can be effectively used in a HDD magnetic reproducing head for reproducing very high-density magnetic recording media. Also, such a ferromagnetic tunneling effect can be used in a magnetic field measuring device for motors, a magnetism sensor such as a geomagnetism sensor for navigation, or a magnetic solid-state memory device that is generally referred to as MRAM.
Japanese Laid-Open Patent Application No. 11-161919 (Japanese Patent No. 3004005) discloses such a TMR device that achieves an improved magnetostatic interactive operation.
Japanese Laid-Open Patent Application No. 5-157566 discloses a general MR device that is used in a direction indicator. To realize a higher magnetic field sensitivity and hysteresis, an auxiliary magnetic field is generated in the MR device.
In Japanese Laid-Open Patent Application No. 11-161919 (Japanese Patent No. 3004005), however, the magnetostatic interactive operation is improved by the lamination structure of the layers, which is not an essential solution and leads to higher production costs.
Also, to produce a direction sensor, it is not desirable to increase the sensitivity by generating an auxiliary magnetic field in an MR device.
In view of these facts, the conventional magnetism sensors are not effective enough in terms of size, weight, costs, and sensitivity, and should be further improved.
An object of the present invention is to provide a tunneling magnetoresistive effect device that is small and light, and has a high sensitivity.
Another object of the present invention is to provide a direction sensor system that can increase the precision in sensing geomagnetism by virtue of the above tunneling magnetoresistive effect device, and can be effectively used in a navigation system or the like.
The above objects of the present invention are achieved by a tunneling magnetoresistive effect device that includes: a soft magnetic layer for assisting magnetic field sensing operations, the soft magnetic layer being stacked on a surface of a substrate; a first spin polarization layer that is stacked on the soft magnetic layer, and has a higher coercive force and a higher spin polarization rate than the soft magnetic layer; a tunneling layer that covers the soft magnetic layer and the first spin polarization layer, and is made of an insulating material or a dielectric material; and a second spin polarization layer that is stacked on the tunneling layer, and corresponds to the first spin polarization layer. In this tunneling magnetoresistive effect device, a magnetism sensing unit is formed by the lamination structure consisting of the soft magnetic layer, the first spin polarization layer, the tunneling layer, and the second spin polarization layer. The thickness of the first spin polarization layer is smaller than the thickness of the tunneling layer or 2 nm, whichever is smaller.
Since any tunneling magnetoresistive effect device is manufactured by a thin-film formation technique, the size and weight of the device can be sufficiently reduced. Also, as the thickness of the first spin polarization layer having a higher coercive force and a higher spin polarization rate than the soft magnetic layer, on which the first spin polarization layer is stacked, is adjusted so as to achieve both a low coercive force and a desirable TMR ratio, the sensitivity for a magnetic field can be sufficiently increased. The soft magnetic layer may be located either at the top or at the bottom of the device.
In the above tunneling magnetoresistive effect device, the thickness of the soft magnetic layer is 10 or more times greater than the total thickness of the tunneling layer and the first spin polarization layer.
With the thickness of the soft magnetic layer being 10 or more times greater than the total thickness of the tunneling layer and the first spin polarization layer, the TMR ratio can be further increased, and the coercive force of the device can be further reduced.
In the above tunneling magnetoresistive effect device, the area of a non-junction part of the soft magnetic layer is 10 or more times greater than a junction area through which tunnel current flows. The junction area is defined by the soft magnetic layer, the first spin polarization layer, and the tunneling layer.
With the area of the soft magnetic layer excluding the junction part being 10 or more times greater than the area of the tunnel-current flowing junction area, the coercive force of the device can be further increased, while the TMR ratio remains unchanged.
The above tunneling magnetoresistive effect device may further include a high permeability layer that is placed in the vicinity of the magnetism sensing unit, and is connected to the soft magnetic layer.
Since the high permeability layer connected to the soft magnetic layer is located in the vicinity of the magnetism sensing unit, the high permeability layer can function as a magnetic flux sink, and thus further increases the sensitivity of the device.
The above tunneling magnetoresistive effect device may further include a bulk magnetic member that is placed in the vicinity of the magnetism sensing unit.
The bulk magnetic member can achieve a much lower permeability and a much lower coercive force than a thin magnetic film. Also, the bulk magnetic member characteristically tolerates a large amount of magnetic flux. With this bulk magnetic member being located in the vicinity of the magnetism sensing unit, the sensitivity of the device can be further increased.
The above tunneling magnetoresistive effect device may alternatively include a bulk magnetic member that is placed on the high permeability layer.
As the bulk magnetic member is formed on the high permeability layer that is located in the vicinity of the magnetism sensing unit, the sensitivity of the device can be further increased.
In the above tunneling magnetoresistive effect device, the soft magnetic layer may have a lamination structure that has a plurality of soft magnetic films stacked on a non-magnetic layer.
As the soft magnetic layer is formed by the lamination structure that has a plurality of soft magnetic films stacked on a non-magnetic layer, the formation of reflux magnetic domains can be prevented. Accordingly, the device causes less noise and enables higher frequency operations (i.e., high-speed sampling operations). Thus, the device capacity can be further increased.
In the above tunneling magnetoresistive effect device, the soft magnetic layer may have a circular shape or a ring-like shape in a plan view.
As the soft magnetic layer has a circular (or oval) shape or a ring-like shape in a plan view, the magnetostatic energy can be sufficiently reduced. Accordingly, less magnetic charge is generated, and magnetic domains can be stabilized. As a result, the device has less noise, and achieves a higher sensitivity. In this manner, the device capacity can be further increased.
In the above tunneling magnetoresistive effect device, the soft magnetic layer may be formed by a plurality of divisional parts.
As each of the divisional parts is smaller than the whole soft magnetic layer, the formation of reflux magnetic domains can be restrained, and noise can be reduced. Also, matched anisotropies can be achieved, and the sensitivity of the device can be further increased.
In the above tunneling magnetoresistive effect device, the soft magnetic layer may have a plurality of notches formed therein.
As the soft magnetic layer is partitioned by the plurality of notches, the formation of reflux magnetic domains can be restrained by virtue of the small size of each divisional part of the soft magnetic layer. Also, noise can be reduced, and matched anisotropies can be achieved. Accordingly the sensitivity of the device can be further increased.
The above tunneling magnetoresistive effect device may further include a reset magnetic field generator for resetting the magnetized state of the magnetism sensing unit to a predetermined state.
Even if an incorrect sensing operation is conducted due to a temporary ferromagnetic field that magnetizes the magnetism sensing unit and shifts the operation point, the reset magnetic field generator resets the magnetized state of the magnetism sensing unit to the predetermined state. By doing so, the reset magnetic field generator enables a normal sensing operation after the resetting.
In the above tunneling magnetoresistive effect device, the reset magnetic field generator may include a reset electric current distributor that is integrally formed in the vicinity of the magnetism sensing unit.
With the reset electric current distributor being integrally formed in the vicinity of the magnetism sensing unit, the resetting operation described above can be more easily realized.
In the above tunneling magnetoresistive effect device, the reset magnetic field generator may alternatively include an external coil for generating a reset magnetic field in the magnetism sensing unit.
With the external coil that generates a reset magnetic field in the magnetism sensing unit, the resetting operation described above can be more easily realized.
The objects of the present invention are also achieved by a direction sensor system that includes: a plurality of tunneling magnetoresistive effect devices that are of any type described above, are independently arranged in the directions of three or more axial vectors, and sense geomagnetism; a sensing unit for sensing three or more axial vectors from sensor outputs of the plurality of tunneling magnetoresistive effect devices; an abnormality sensing unit for determining whether an abnormality exists in a sensed result by comparing the absolute value of each of the sensed results of the tunneling magnetoresistive effect devices with a predetermined threshold value; and an alarm unit for notifying that an abnormality exists in the sensed result, when the abnormality sensing unit senses the abnormality.
In this direction sensing system that senses geomagnetism, the three or more axial vectors are detected based on the sensor outputs of the tunneling magnetoresistive effect devices that are independently arranged in the directions of three or more axial vectors. Here, the absolute value of each of the sensor outputs of the tunneling magnetoresistive effect devices is compared with the threshold value that is defined by adding the measurement margin to the measured geomagnetism strength, so as to determine whether an abnormality exists in a sensed result. If an abnormality is detected, the alarm unit notifies the system user of the existence of the abnormality, so that the use of the incorrect sensed result can be prevented.
The objects of the present invention are also achieved by a direction sensor system that includes: a plurality of tunneling magnetoresistive effect devices that are of any of the above described types including a reset magnetic field generator for resetting the magnetized state of the magnetism sensing unit to a predetermined state, are independently arranged in the directions of three or more axial vectors, and sense geomagnetism; a sensing unit for sensing three or more axial vectors from sensor outputs of the plurality of tunneling magnetoresistive effect devices; an abnormality sensing unit for determining whether an abnormality exists in a sensed result by comparing the absolute value of each of the sensed results of the tunneling magnetoresistive effect devices with a predetermined threshold value; an alarm unit for notifying an abnormality exists in the sensed result, when the abnormality sensing unit senses the abnormality; and a resetting unit for providing reset electric current to the reset magnetic field generator so as to reset the magnetized state of the magnetism sensing unit to a predetermined state, when the abnormality sensing unit senses an abnormality.
If an abnormality is detected in a sensed result, the resetting unit provides reset electric current to the reset magnetic field generator, so as to reset the magnetized state of the magnetism sensing unit to the predetermined state. After this resetting operation, further incorrect sensing operations can be prevented.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.